DESCRIPTION (Verbatim from the Applicant): In the clinical setting, areal bone mineral density (BMD) measurements by dual x-ray absorptiometry (DXA) estimate hip fracture risk by providing a surrogate measure for proximal femoral strength. Because DXA is a planar imaging modality, it has potentially important limitations. First, DXA quantifies integral bone mass and density, whereas the influence of the cortical and trabecular compartments on mechanical strength may exceed their contributions to the integral bone mass. Second, DXA BMD measurements scale with bone size; larger bones appear to be denser. The overall goal of our study is to understand how the projectional nature of DXA imaging affects its ability to depict proximal femoral strength in two populations with known differences in hip fracture rates, volumetric BMID, bone size and cortical thickness. To achieve this goal, we will carry out a study in vivo comparing proximal femoral strength, compartmental BMD and geometry in 400 elderly Caucasian and African-American women. To estimate femoral strength, we will image the subjects with volumetric quantitative computed tomography (vQCT) of the proximal femur and construct finite element (FE) models using the vQCT scans. We will calculate proximal femoral failure load (FL) by loading these models to failure with forces simulating a fall to the side with impact on the posterolateral aspect of the greater trochanter and a joint reaction force simulating a spontaneous fracture. In order to determine the relationship between DXA BMD and FL in these two race groups, we will acquire DXA hip scans in all of the subjects. To determine the relationship between FL, compartmental BMD and 3-D bone geometry, we will analyze the vQCT scans with a computer algorithm, which extracts measures of trabecular/cortical BMD and cross-sectional geometry. By determining the 3-D geometric and/or BMD factors which result in greater proximal femoral bone strength and understanding how these measures are modified by planar projection, we will gain considerable insight into the performance of DXA, the principal technique used to estimate proximal femoral strength and therefore, hip fracture risk.